1. Field of the Invention
The present invention relates in general to mounting devices for mounting an automotive power unit or the like and more particularly to power unit mounting devices of a type which is equipped with an active damping actuator.
2. Description of the Prior Art
One of power unit mounting devices the above-mentioned type is disclosed in Japanese Patent First Provisional Publication No. 59-23140. In the device of this publication, for absorbing vibrations caused by a power unit, there is employed a unit of laminated electrostrictive elements whose thickness varies in accordance with a sine-wave voltage applied thereto. The unit is disposed between the power unit and a support structure in such a manner that the vibrations of the power unit can be cancelled by the repeated variations in thickness of the unit of laminated electrostrictive elements. That is, the unit of laminated electrostrictive elements and the power unit are so arranged that the direction in which the thickness of the unit of electrostrictive elements varies is consistent with the direction in which the power unit vibrates. Therefore, vibrations transmitted from the power unit to the support structure can be reduced to a minimum level.
In fact, the power unit mounting device of the publication has such a structure that the unit of laminated electrostrictive elements is sandwiched between upper and lower plates which are connected through connecting bolts and nuts. The upper plate is secured to the power unit and the lower plate is connected through a rubber vibration insulator to the support structure.
However, in this known mounting device, due to its inherent construction, the expansion and contraction of the unit of laminated electrostrictive elements cause repeated applications of stress to the connecting bolts. This tends to shorten the life of each bolt.
Furthermore, in this known mounting device, the connecting bolts are quite strongly fastened by the nuts for assuredly holding the unit of laminated electrostrictive elements even under the contraction condition of the same. However, this tight fastening of the bolts prevents the unit of laminated electrostrictive elements from achieving a sufficient expansion, and thus, the vibration damping effect of the device is poor.